The present invention relates to a frequency sweep control apparatus and, more particularly, to a frequency sweep control apparatus in a radio communication receiver used for satellite communications and the like, which apparatus is designed to demodulate an optimal reception signal while performing sweep control of a local oscillation frequency.
In a demodulation apparatus of this type, an AFC (Auto Frequency Control) operation conforming to variations in the frequency of a reception signal upon synchronization with the reception signal and a sweep operation to repeatedly sweep the variable frequency range of the reception signal until establishment of synchronization with the reception signal are selectively performed in accordance with synchronous and asynchronous states with respect to the reception signal.
In a demodulator, in order to demodulate a reception signal, the frequency of the reception signal and the frequency of an oscillator in the demodulator must be set to coincide with each other. The frequency of the reception signal, however, is shifted near a certain frequency (center frequency) due to a frequency deviation caused in a satellite as a repeater in satellite communications.
Especially in satellite communications, since the frequency in use is set on the order of GHz, even a small frequency variation causes a reception intermediate frequency to greatly shift from a true value when it is input to a demodulator. If the frequency in use is, for example, 12 GHz, a 1-ppm variation corresponds to a 12-kHz variation.
Sweep operations are performed while the frequency is gradually changed until the demodulator can demodulate the reception signal. Every time a sweep operation is performed, demodulation success/failure determination is performed. For example, demodulation success/failure determination is based on whether a unique word signal (or synchronization signal) detector detects a unique word signal. Methods of sweep include a reciprocal sweep method in which the frequency is alternately changed from the minimum value to the maximum value and from the maximum value to the minimum value, and a cyclic sweep method in which the frequency is repeatedly changed from the minimum value to the maximum value.
FIG. 1 shows a conventional frequency sweep control apparatus of this type. Referring to FIG. 1, reference numeral 1 denotes a variable frequency oscillator; 2, a demodulator for demodulating a reception IF (Intermediate Frequency) signal by using an output from the variable frequency oscillator 1, and outputting phase error information based on the difference between the reception IF signal and the frequency of the variable frequency oscillator 1; 3, a unique word signal detector 3 for detecting a unique word signal in a demodulated signal output from the demodulator 2; 4, a phase error detector for detecting a phase error from the phase error information output from the demodulator 2; and 5, a frequency controller for controlling the oscillation frequency of the variable frequency oscillator 1 on the basis of the outputs from the unique word signal detector 3 and the phase error detector 4. An LPF (low-pass filter) 7 is inserted in a control signal line of the frequency controller 5 so that a control voltage is output to the variable frequency oscillator 1 through the LPF 7. The time constant of the LPF 7 is fixed.
When a unique word signal is detected by the unique word signal detector 3, it is determined that synchronization is established. At this time, the oscillation frequency of the variable frequency oscillator 1 is controlled by a signal output from the phase error detector 4 through the frequency controller 5 and the LPF 7. This operation is an Auto Frequency Control (AFC) operation. If no unique word signal is detected by the unique word signal detector 3, asynchronization is determined. In this case, the frequency controller 5 performs control to sweep an output from the variable frequency oscillator 1. The reception IF signal is a modulated signal, e.g., a PSK-modulated signal. The demodulated signal is a two-component system constituted by an in-phase component and an orthogonal component.
Note that the unique word signal detector 3 is designed to output a signal indicating whether a unique word signal (unique word) contained in a reception signal is detected. When the unique word signal detector 3 detects a unique word signal, it means that the demodulator has properly demodulated the reception signal. That the demodulated reception signal is output means that all the synchronization required for a receiving operation, i.e., carrier synchronization, phase lock, clock synchronization, and bit synchronization, has been established. The frequency controller 5 is designed to control the frequency of the oscillator in the demodulator. The frequency controller 5 operates in both the sweep mode and the AFC mode. In the sweep mode, the oscillation frequency is changed stepwise. In the AFC mode, the oscillation frequency is continuously changed. Discrimination between a sweep state and an AFC state is dependent on an output from the unique word signal detector.
In the conventional frequency sweep control apparatus having such an arrangement, however, the sweep range of the reception system is defined by the sum of maximum frequency shifts in the overall apparatus, determined at the stage of system design, e.g., the maximum on the transmission side and the maximum frequency shift in a repeater, and the entire sweep range is always swept by the frequency controller 5 in the sweep mode. For this reason, even when a break occurs, e.g., when the currently used system is switched to a backup system on the transmission side, detection of a unique word signal cannot be performed on the reception side until one sweep cycle is completed to restore the oscillation frequency of the variable frequency oscillator 1 to the initial frequency. As a result, it takes much time to reestablish the synchronization.
In addition, although the oscillation frequency of a VCXO (Voltage-Controlled Crystal Oscillator) generally used as a local oscillator for demodulation can be changed by changing an externally applied control voltage, the precision of the variable frequency function of the VCXO is poor, and the oscillation frequency greatly changes with temperature and time. Therefore, it is technically impossible to uniquely determine the relationship between an input control voltage and an output oscillation frequency. FIG. 2 shows changes in the input/output characteristics of the VCXO in this case.
In the conventional demodulation apparatus shown in FIG. 1, which uses the VCXO having such characteristics as a local oscillator, since the control voltage is changed by the frequency controller 5 while the state of change in local oscillation frequency with temperature and time is left indefinite, the actual frequency sweep range changes.
For this reason, as shown in FIG. 3 the VCXO can be used only when the difference between the minimum necessary sweep range (defined as .+-.20 kHz) and the maximum allowable sweep range (defined as .+-.80 kHz) is large. In this case, the minimum necessary sweep range is a range determined by the sum of the frequency shift on the transmission side and the frequency shift in the repeater. The maximum allowable sweep range means the maximum sweep range in which demodulation of a signal from another radio communication system is reliably inhibited.
If, for example, the frequency shift in the transmitter is 5 kHz, and the frequency shift in the repeater is 15 kHz, the minimum necessary sweep range is 5+15=20 kHz. If the frequency spacing is 100 kHz, the maximum allowable sweep range is 100-20=80 kHz.
Assume that the difference between the minimum necessary sweep range and the maximum allowable sweep range is small, as shown in FIG. 4A. In this case, if a VCXO having characteristics A and B, as shown in FIGS. 4B (reference symbol C in FIG. 4B denotes the output control voltage range of the frequency controller 5), the sweep ranges shown in FIG. 4A are respectively set. That is, with the characteristics B, sweep is also performed outside the sweep range, and hence signals other than a target signal may be received.
In addition, in such a conventional frequency sweep control apparatus, the time constant of the LPF 7 is preferably set to be small in the sweep mode of the frequency controller because the frequency is changed stepwise, whereas in the AFC (Auto Frequency Control) mode in which phase synchronization has been established, the time constant is preferably increased to some extent because the BER (Bit Error Rate) of the demodulator is increased with an abrupt change in the oscillation frequency of the variable frequency oscillator. For this reason, in the conventional frequency sweep control apparatus in which the time constant of the LPF 7 is fixed, either the synchronization establishment characteristic or the operation stabilization characteristic must be sacrificed, or both the characteristics must be sacrificed to some extent. Therefore, satisfactory characteristics cannot be obtained.
Furthermore, since the gain of a loop filter in a demodulator generally changes with a change in the C/N (carrier-to-noise) ratio, the lock-in range (the difference between a reception intermediate frequency and the frequency of a variable frequency oscillator) in which the demodulator can demodulate the reception IF signal is dependent on the C/N ratio of the reception IF signal. That is, the range is expanded with an increase in C/N ratio, and vice versa.
In the conventional frequency sweep control apparatus, as shown in FIG. 5, the frequency step width in the sweep mode is fixed to a certain value determined on the basis of the lock-in range in which demodulation can be performed by the demodulator even with a low C/N ratio.
If the frequency step width in the sweep mode is set to be small to enable reception synchronization even with a low C/N ratio, the number of steps required to cause the oscillation frequency of the variable frequency oscillator to fall within the range of allowable frequency differences from the frequency of the reception IF signal is increased. In the sweep mode, every time the oscillation frequency of the variable frequency oscillator is changed, the demodulator must wait for a predetermined period of time to check whether synchronization with the reception IF signal is established. Therefore, the sweep time is prolonged with an increase in the number of steps.
In contrast to this, assume that the frequency step width is set to be optimal when the C/N ratio is high. Note that the C/N ratio is high in a nominal operating state in satellite communications. For example, if the link budget of satellite communications is set to be 99.95% for BER (bit error rate) .ltoreq.1.0E-6, the duration of "high C/N ratio" occupies 99% or more of the total operating time. In this case, the difference between the carrier frequency of the reception IF signal and the oscillation frequency of the variable frequency oscillator is always larger than the lock-in range in which the demodulator can demodulate the reception IF signal with a low C/N ratio. For this reason, demodulation may not be performed.